Tentering frames for the stated purpose include tentering chains and at least one guide rail that supports and guides the respective tentering chain along its endless path. Each tentering chain is equipped with tentering clamps having grippers for gripping the margin or edge of a film web. The tentering clamps are carried by chain links interconnected to form the tentering chain. Each clamp is provided with guide elements, for example, guide rollers that travel along the guide rail. The spacing between two neighboring tentering clamps is continuously adjustable for varying the pitch that corresponds to the just mentioned spacing.
The continuous pitch variation may, for example, be accomplished by kinking neighboring chain links relative to each other so that they deviate from a straight line. European Patent Publication EP 0,291,775 A2 corresponding to U.S. Pat. No. 4,890,365 (Langer) issued on Jan. 2, 1990, discloses an apparatus for treating film webs simultaneously in two axial directions. The pitch or mutual spacing between neighboring tentering clamps in the tentering chain according to Langer is continuously variable. For this purpose, the tentering chain is equipped with so-called kinking chain links which in their stretched-out condition travel substantially along a straight line in a common plane. However, when the chain links are kinked, they do not travel any more in a common plane. Rather, the chain links then form a zig-zag configuration.
In the Langer disclosure it is necessary that an intermediate chain link is inserted between neighboring tentering clamps so that a deflection of the chain links between neighboring clamps becomes possible at all. The intermediate chain link articulates neighboring kinking chain links to each other, whereby the kinking becomes possible. The known tentering chain is so constructed that the forces necessary for varying the pitch between neighboring tentering clamps are effective on the tentering clamps and must be transmitted through the intermediate chain link into the guide mechanism of the tentering clamp. Since the tentering clamp itself forms a rigid portion of the tentering chain and since the tentering clamp itself is also rigid, the adjustment of the pitch can only be accomplished through the intermediate chain link arranged between neighboring clamps. This features permits relatively small pitch variations because an adjustment of the tentering clamps relative to each other is achieved only by a relative small deflection of the intermediate chain link that articulates two neighboring tentering clamps to each other. This limitation is due to the fact that the journal axis of two neighboring tentering clamps is not located in a clamp body itself, but rather in a journal joint outside of a tentering clamp body.
As a result, the on-center spacing between neighboring tentering clamp bodies is relatively large so that the corresponding pitch is also relatively large and any variation of the large pitch is relatively small, which is not desirable. Thus, there is room for improvement.